1. Field of the Invention
The field of art to which this invention pertains is hydrocarbon processing. Specifically this invention relates to a fluid catalytic cracking process in which hydrocarbon feed streams comprising residual oils are converted to distillate fuels.
2. Prior Art
The fluid catalytic cracking process has for many years been considered one of the major gasoline producers for the refining industry. Typically the feedstocks employed have been gas oils and operating conditions have been selected for relatively high conversion of the gas oil feedstocks to permit maximum yields of gasoline. The cracking of gas oils to gasoline is fairly well understood and the limitations imposed on the feedstock gas oils are quite well defined. The major limitations currently placed on gas oil feedstocks are the amount of carbon or "coke" precursors and the amount of metallic compounds contained in the feedstocks.
These coke formers or precursors, which are typically high molecular weight condensed ring hydrocarbons, are primarily a function of the crude type (aromatic, naphthenic, or paraffinic) and the boiling range of the material. Results obtained by Conradson or Ramsbottom coke analyses represent approximate measures of these compounds and of the coking propensity of the raw oil. Normally it is desirable to limit the gas oil feedstocks to 0.5 wt. % or less by the Conradson method. The reason is that circulating cracking catalyst is an excellent coke remover and as the coke precursor compounds in the gas oil are permitted to increase much more coke is deposited on the catalyst in the hydrocarbon reaction zone than is required for the process heat balance. Temperatures in the regeneration zone, where the coke is oxidized from the catalyst, can become excessive and, equally important, the catalyst particle temperatures can rise to the point that the catalytic structure is damaged or destroyed with a resulting loss in activity.
The second major limitation imposed on gas oils is the content of organic nickel, vanadium, and iron compounds. These materials, commonly called "porphyrins," are distilled into the high boiling fractions of vacuum gas oils. Typically, the metal content is limited to the extent that the metals factors, defined by the equation F.sub.m = (10 wt-ppm Ni) + wt-ppm (V + Fe), is less than 2.5. As with the carbon precursors, the circulating catalyst adsorbs these metals almost completely and becomes "poisoned." These metals, in their active state on the catalyst, depress the yield of primary gasoline product, promote various dehydrogenation reactions, and can produce large quantities of hydrogen and coke. This can lead to tremendous increases in volumes of unwanted gases and very quickly overload gas compressors and gas recovery facilities.
When one compares residual oils such as whole crudes or atmospheric reduced crudes to typical gas oils, it is apparent that in addition to being more difficult if not impossible to vaporize completely, the residual oils contain higher amounts of "coke"]precursors (as determined by coke analyses) and larger quantities of metals Ni, V, and Fe. In spite of the potential processing difficulties imposed by the presence of such contaminants, the refiner has been prompted by the tightening of crude supplies to expand the characteristics of the feedstock charged to the FCC process beyond those of the relatively clean gas oils presently being processed. In order to increase charge stock availability to meet gasoline and fuel oil demands higher-boiling feeds which were previously considered only marginal or unsuitable because of such contaminants are receiving substantial considerations.
Although my invention is concerned with the processing of these more contaminated feedstocks, no novelty is asserted in the processing of such feedstocks in a conventional FCC process. While such processing in a conventional FCC process is not common, a relatively few operations throughout the world have been processing feedstocks containing reduced crudes on a daily basis. Likewise no novelty is asserted in any of the individual components of my process; rather my invention resides in a novel combination of a riser reaction zone and a CO-burning regenerator for processing, in a more efficient and economical manner feedstocks comprising residual oils to produce distillate fuels. In a preferred embodiment my invention permits the processing of the more typical reduced crudes which contain more Conradson carbon and more metal contaminants than do the conventional gas oils and typical reduced crudes now being processed.